This invention pertains to the field of optical sensors or detectors generally and, in particular to that class of optical sensors which are non-imaging. Specifically, it pertains to techniques for controlling the field-of-view of a non-imaging optical sensor or detector.
Traditional methods for controlling the field-of-view of an optical sensor or detector involve complex arrangements or lenses or field stops. One approach is to design a lens with three or more elements, whose relative positions are adjusted to affect a change in the focal length of the lens, and hence the field of view. Another approach is to design a lens having a constant focal length, placing a variable aperture at the focal plane of the lens to serve as a field stop. Both of these approaches encounter practical difficulties for a large field-of-view. When the field-of-view exceeds 60.degree. full angle, the physcial aperture of the lens exceeds the pupil, or optical aperture, of the sensor. For example, a lens system with a 190.degree. field-of-view with a pupil of ten inches (10") would be twenty-one feet (21') in diameter and sixty feet (60') long.
There is a class of optical applications that does not require the use of imaging detectors or sensors. A laser communications system using an atomic resonance filter (ARF) is an example of non-imaging optical system. The atomic resonance filter (ARF), comprised of vapors of specific atoms, has been developed recently as a new type of narrow band optical filter. (Cf. "Atomic resonance filers", Jerry A. Gelbwachs, IEEE Journal of Quantum Electronics, Vol. 24, No. 7, July 1988, and U.S. Pat No. 4,829,597). The atomic resonance filter is an optical filter which does not preserve the optical coherence of the incoming light and, consequently, all imaging qualities are lost in such a filter. Thus, there is no specific advantage to using an imaging system for defining the field-of-view for an optical detector using an atomic resonance filter. This is particularly true when large apertures are needed to collect weak signals over very wide fields-of-view.